The perfluoroalkyl chain monomers are also used for generating hydrophobic surfaces from pulsed plasma deposition processes (see WO 9858117 A1).
The power of the plasma initiated polymerisation affects the nature of the polymer produced. The higher average energy inputs of continuous wave plasmas lead to more fragmentation of the monomer, and so the polymer loses structural properties of the monomer. In the case of 1H,1H,2H,2H-perfluorodecyl acrylate (PFAC8), there's less retention of the perfluoroalkyl chain and the contact angle of the surface coating is compromised. Higher plasma energies also lead to more crosslinking. For the lower average energy inputs of pulsed plasmas, there's better retention of monomer structure and less crosslinking. The greater retention of the perfluoro chain under low energy, pulsed plasma conditions leads to best levels of contact angles for the surface coating.
When the perfluoroalkyl chains have eight or more fluorinated carbons (long chain), the polymer made from the monomer has a crystalline structure. When the perfluoroalkyl chains have less than eight fluorinated carbons, the resulting polymer is amorphous and so can be unstable in the presence of water (see “Molecular Aggregation Structure and Surface Properties of Poly(fluoroalkyl acrylate) Thin Films, Marcomolecules, 2005, vol 38, p 5699-5705) When long chain perfluoroalkyl polymers are produced by high average power (continuous wave or CW) or low average power (pulsed wave or PW) plasmas, then because of the crystalline structure of the long chains, the polymers are non-stick to the touch and stable in the presence of water.
However, the feel and water stability of shorter chain polymer coatings is affected by the plasma power levels used. For example, when PFAC6 (1H,1H,2H,2H-perfluorooctyl acrylate) is polymerised in low power plasma conditions, the resulting polymer coating can have several disadvantages. For example the coating can cause water drops to spread out a little (slump), be marked by the presence of a water drop on its surface, have a tacky feel, or can be easily smeared (for example on substrates of silicon wafer and ABS plastic).
By increasing the power of the plasma used for polymerisation, the polymer becomes more crosslinked and becomes more resistant to smearing. However, increasing the power has the concomitant effect of decreasing the water contact angle through more monomer fragmentation (as described above). FIG. 1 shows the effect of increasing the power to monomer flow ratio for CW plasmas in a 125 liter chamber: at a ratio of 4 W/μl/min, the water contact angle is ˜85-95 degrees and the coating is tack-free. However, as the ratio drops, the contact angle increases and the occurrence of tacky/smudgy coatings increases too. FIG. 2 shows the same effect for pulsed plasma conditions. These results show that the process window for producing tack and smudge-free coatings have a limited plasma processing range and the final coating has a compromised water contact angle.
Accordingly, it is an aim to solve one or more of the above-mentioned problems with the prior art coatings.